In spread spectrum communications, such as in CDMA systems, pseudorandom noise (PN) sequences are used to generate spread spectrum signals by increasing the bandwidth (i.e., spreading) of a baseband signal. A forward link waveform transmitted by the base station may be comprised of a pilot waveform and a data waveform. Both of the waveforms are received with the same relative phase and amplitude distortions introduced by the channel. The pilot waveform is an unmodulated PN sequence which aids in the demodulation process, as is well-known in the art as “pilot-aided demodulation.” Conventional pilot-aided demodulation methods typically include the steps of (i) demodulating the pilot waveform, (ii) estimating the relative phase and amplitude of the pilot waveform, (iii) correcting the phase of the data waveform using the estimated phase of the pilot waveform, and (iv) adjusting the weight of data symbols used in maximal ratio combining in a RAKE receiver based on the estimated amplitude of the pilot waveform. Steps (iii) and (iv) above are performed as a “dot product” as is known in the art. Conventionally, steps (i) through (iv) are performed in hardware. In other conventional methods, a controller having a central processing unit (CPU) and/or a digital signal processor (DSP) may perform some of the above-described steps.
FIG. 1 illustrates a conventional IS-95A or TIA/EIA-95-B forward link base station transmitter 10 (prior art). A pilot channel 12 is generated that has no data. That is, the data is predetermined to be all “0” bits. The pilot channel is modulated, or covered with a Walsh code from Walsh code generator 14 at 1.2288 Mcps (megachips per second). 64 orthogonal Walsh codes, each of 64 bits, are used in the IS-95A and TIA/EIA-95-B systems. Walsh code H0 is used to modulate the pilot channel.
Also depicted is a traffic or paging channel, which shall be referred to herein as an information channel. Data is input at one of a plurality of data rates from 9.6 kbps (kilobits per second) to 1.2 kbps. The data is encoded at encoder 16, at one bit per two code symbols, so that the output of the encoder 16 varies from 19.2 ksps (kilosymbols per second) to 2.4 ksps. Symbol repetition device 18 repeats the code symbols from 1 to 8 times to create a 19.2 ksps signal. Alternately stated, either 1, 2, 4, or 8 modulation symbols are created per code symbol. Then, the information channel is scrambled with a long code at the same 19.2 ksps rate. Other rates are described in the IS-2000 standard. The information channel is covered with a different Wash code from that used to cover the pilot channel, code HT for example.
After being modulated with Walsh codes, each channel is spread with a common short code, or PN sequence. Each channel is split into I and Q channels, and spread with I and Q channel PN sequences. A 90 degree phase shift is introduced by multiplying the I channels with a sin function, while the Q channels are being multiplied with a corresponding cosine function. Then, the I and Q channels are summed into a QPSK channel. In the IS-95A and TIA/EIA-95-B standards, the same baseband symbols are assigned to both the I and Q channels. The combination of all the QPSK channels, including pilot, synchronization, paging, and traffic channels can be considered a composite waveform. This composite waveform is then up-converted in frequency (not shown) and transmitted.
FIG. 2 is a conventional IS-95A or TIA/EIA-95-B CDMA receiver (prior art). At the mobile station receiver 50 the transmitted signals are accepted as analog information, and converted into a digital I and Q sample stream at A/D 52. Conventionally, a multi-finger RAKE is used to variably delay and amplify multipath delays in the sample stream, so that degradation due to fading can be minimized. Three demodulating fingers, demodulating finger 1 (54), demodulating finger 2 (56), and demodulating finger 3 (58) all receive the same I and Q sample stream, which has been represented as a single line for simplicity. Each demodulating finger is assigned one of the sample stream multipath delays. PN codes and Walsh codes are generated with delays consistent with the multipath delays of the sample stream to be demodulated. The sample stream from the multipaths is coherently combined in combiner 60 based on a maximal ratio combining (MRC) principle.
The IS-2000 standards propose, and future uses will include multiple information channels with a variety of symbol rates. A variety of symbol accumulation periods will be required in the process of demodulating these information channels. In IS-95A and TIA/EIA-95-B standard communications, a symbol is conventionally spread with 64 PN chips at the transmitter. At the receiver, the symbol is recovered by despreading, uncovering, and accumulating the symbol over a period of 64 PN chips. The accumulated symbol is called a soft symbol. Conventionally, the soft symbol is corrected with respect to phase and weighted with respect to amplitude after accumulation, using the pilot waveform as a phase and amplitude reference.
The receiver 50 may also receive a sample stream including signals from more than one base station. The base stations are precisely timed and synchronized using offsets of the PN spreading code. That is, the sample stream received from two different base stations has delays that are typically much larger than multipath delays. The receiver 50 has diversity characteristics which permit it to demodulate the sample stream from multiple base stations, for the purpose of a handoff for example.
In some conventional CDMA RAKE receivers, the outputs of multiple demodulating fingers are “hardwired” to combine the common information channels in a sample stream. The decision and data transfer operations of the individual finger channels are predetermined. Hardwiring reduces flexibility, as the finger channels of the demodulating fingers must always be combined with the same partner finger channels. Thus, the number of information channels, the information channel order, and the information channels that can be combined across demodulating fingers are necessarily constricted when the finger channel outputs are connected in a hardwired arrangement. Hardwiring does not permit partner finger channels to be used with different combiner channels. A conventional receiver with a fixed number of finger channels in each demodulating finger can only demodulate such a fixed number of IS-2000 standard information channels.
Alternately, the soft symbols output by the demodulating finger can be buffered and transferred, via a data bus, to a CPU or DSP for combining. This software combining approach provides flexibility, as potentially the finger channels can be combined in any variation. However, the CPU or DSP may not have enough bandwidth to perform the combining operations, nor will such solutions prove power efficient.
To increase the throughput of information in high speed data links, the IS-2000 standards also propose the use of associated information channels that are generated in the transmitter from a single information stream through various demultiplexing methods, for example, QPSK, OTD (orthogonal transmit diversity), and multicarrier modes. The simplest example is the demultiplexing of a single information stream into 2 BPSQ channels transmitted as a QPSK channel.
An IS-2000 receiver should, therefore, receive and demodulate multiple carrier signals, and the corresponding sample streams, as well as process and combine associated information channels. Since size and power consumption are always a serious concern in the design of mobile station receivers, the complexity of the new IS-2000 standard presents the designers with the challenge of expanding receiver capabilities without dramatically increasing the receiver complexity and power consumption.
It would be advantageous if a CDMA receiver could be designed to permit cooperation between demodulating fingers, so that associated information channels from orthogonal sample streams could be efficiently demodulated. It would also be desirable if the soft symbols generated from the associated information channels being demodulated in separate finger channels and separate fingers could be efficiently multiplexed back into a single information channel.
It would be advantageous if the number of demodulating fingers, and the number of finger channels in a demodulating finger that are required to demodulate associated information channels in either the same, or orthogonal sample streams, could be minimized. Such efficient processing of associated information channels would permit the receiver to demodulate a greater number and variety of channels.